This invention relates to a method of fabricating a contact device.
It is known to test an integrated circuit having an array of connection pads at the wafer stage of fabrication, i.e., before dicing into separate chips, using a flexible wafer probe, comprising a sheet of polyimide having contact bumps projecting from one surface thereof in an array corresponding to the array of connection pads of the integrated circuit. The contact bumps are connected to a measurement instrument through strip-form signal lines deposited on the polyimide sheet.
A known method of fabricating a wafer probe is illustrated in FIGS. 1A-1C of the accompanying drawings. A polyimide sheet 2 (FIG. 1A) has a copper substrate 4 bonded to its back surface. A layer 6 of plate-up base is deposited over the front surface 8 of sheet 2 by electron beam evaporation. Layer 6 may comprise successive films of titanium, palladium and gold. Layer 6 is patterned and a layer of gold is deposited electrolytically on the portions of layer 6 that remain, forming contact bases 8 and signal runs 10. A thick layer 14 (FIG. 1B) of photoresist is deposited over the surface of sheet 2 and is exposed to actinic radiation in accordance with the pattern of contact bases 8. The photoresist is developed and the resist that was exposed is removed, forming holes 16 through which the contact bases are exposed. Nickel is deposited electrolytically into holes 16 to form contact bumps 18 (FIG. 1C) bonded to contact bases 8, and the photoresist is stripped. The copper substrate is thinned, and is then patterned and etched to the configuration desired for a ground plane.
The fabrication process described with reference to FIGS. 1A-1C is subject to disadvantage. Since layer 14 is quite thick, it is difficult to ensure that all photoresist overlying contact bases 8 is removed after exposure and development of the photoresist, and consequently the nickel contact bumps normally do not contact the entire area of their respective gold contact bases. Consequently, the strength of the bond between the contact bumps and the respective contact bases is impaired. The holes formed in layer 14 do not have vertical sides, and therefore the bodies of nickel that are deposited into the holes to form the contact bumps are not cylindrical but tend to have a mushroom configuration and to be non uniform in height. In order to ensure reliable contact between the bumps and the connection pads of the circuit under test, the bumps are ground to a uniform height when the ground plane is in engagement with a planar back-up member. This grinding imposes stresses on the bumps, and frequently the bumps are broken from their contact bases, particularly because of the poor adhesion between the bumps and their bases.